1. Field of Invention
This invention relates generally to an electrical interconnection systems and more specifically to improved signal integrity in interconnection systems.
2. Discussion of Related Art
Electrical connectors are used in many electronic systems. It is generally easier and more cost effective to manufacture a system on several printed circuit boards (“PCBs”) which are then connected to one another by electrical connectors. A traditional arrangement for connecting several PCBs is to have one PCB serve as a backplane. Other PCBs, which are called daughter boards or daughter cards, are then connected through the backplane by electrical connectors.
Electronic systems have generally become smaller, faster and functionally more complex. These changes mean that the number of circuits in a given area of an electronic system, along with the frequencies at which the circuits operate, have increased significantly in recent years. Current systems pass more data between printed circuit boards and require electrical connectors that are electrically capable of handling the increased bandwidth.
As signal frequencies increase, there is a greater possibility of electrical noise being generated in the connector in forms such as reflections, cross-talk and electromagnetic radiation. Therefore, the electrical connectors are designed to control cross-talk between different signal paths and to control the characteristic impedance of each signal path. Shield members are often used for this purpose. Shields are placed adjacent the signal contact elements.
Cross-talk between distinct signal paths can be controlled by arranging the various signal paths so that they are spaced further from each other and nearer to a shield, which is generally a grounded plate. Thus, the different signal paths tend to electromagnetically couple more to the shield and less with each other. For a given level of cross-talk, the signal paths can be placed closer together when sufficient electromagnetic coupling to the ground conductors are maintained.
Shields are generally made from metal components. However, U.S. Pat. No. 6,709,294 (the “294 patent”), which is assigned to the same assignee as the present application, describes making shields in a connector from conductive plastic. The '294 patent is hereby incorporated by reference in its entirety.
Electrical connectors can be designed for single-ended signals as well as for differential signals. A single-ended signal is carried on a single signal conducting path, with the voltage relative to a common reference conductor being the signal.
Differential signals are signals represented by a pair of conducting paths, called a “differential pair.” The voltage difference between the conductive paths represents the signal. In general, the two conducing paths of a differential pair are arranged to run near each other. No shielding is desired between the conducting paths of the pair but shielding may be used between differential pairs.
One example of a differential pair electrical connector is shown in U.S. Pat. No. 6,293,827 (“the '827 patent”), which is assigned to the assignee of the present application. The '827 patent is incorporated by reference herein. The '827 patent discloses a differential signal electrical connector that provides shielding with separate shields corresponding to each pair of differential signals. U.S. Pat. No. 6,776,659 (the '659 patent), which is assigned to the assignee of the present application, shows individual shields corresponding to individual signal conductors. Ideally, each signal path is shielded from all other signal paths in the connector. Both the '827 patent and the '659 patents are hereby incorporated by reference in their entireties.
While the electrical connectors disclosed in the '827 patent and the '659 patent and other presently available electrical connector designs provide generally satisfactory performance, the inventors of the present invention have noted that at high speeds (for example, signal frequencies of 1 GHz or greater, particularly above 3 GHz), electrical resonances in the shielding system can create cross talk and otherwise degrade performance of the connector. We have observed that such resonances are particularly pronounced in ground systems having a shield member per signal contact or per differential pair.
My prior patent, U.S. Pat. No. 6,786,771, now published as US 2004/0121652A1, which is hereby incorporated by reference in its entirety, describes the use of lossy material to reduce unwanted resonances and improve connector performance. It would be desirable to further improve connector performance.